This invention relates generally to electronically tunable microwave filters. In particular, it relates to the class of filters utilizing yttrium-iron-garnet (YIG) spheres. Filters of this type are well known in the art, and the invention lies in specific construction details which provide improved filter performance.
Those desiring information on the theory of ferrimagnetic devices are referred to "Principles of Microwave Ferrite Engineering" by J. Helszajn, copyright 1969, John Wiley & Sons, Ltd. The design of YIG filters is described in "Equivalent Circuits Aid YIG Filter Design" by Robert E. Tokheim, Microwaves, April, 1971, pp. 54-59.
Conventional YIG filters utilize a single piece housing in which to mount the YIG spheres, coupling loops, coaxial cables, and their associated mounting parts. Several problems are inherent in this construction method. For example, large (that is, large in relation to the typical coupling loop wire diameter of 0.004 inches) holes must be drilled through the housing in order to permit the coaxial cables to be fed to the YIG sphere cavity. If the transition from the coaxial cable center conductor to a coupling loop is made at the cavity edge, the coupling to spurious magnetostatic modes is greatly increased. If, on the other hand, the transition is made too far away from the edge, inductance is added to the filter causing the input coupling to change as the filter frequency is tuned.
Described in U.S. Pat. No. 3,435,385 is an electronically tunable microwave YIG filter which uses perpendicular loops to couple energy to and away from the YIG spheres, but construction of the housing for the YIG spheres is not described.